The present invention relates to a cathode for a high frequency thermionic tube and more particularly to a thermionic emission cathode with direct heating.
In high frequency thermionic tubes of the triode, tetrode or pentode type having a cathode, an anode and one, two or three grids it is advantageous to make the grids from pyrolytic graphite, a material well known for its mechanical and thermal properties. However, in said same tubes the cathodes are generally in the form of thoriated tungsten filaments for thermionic emissivity reasons. Thus, in operation there are mechanical problems due to the differences in the thermal behaviour of these materials. These problems are only inadequately solved by costly mechanical assemblies or by constraining conditions of using the tubes, such as for example the permanent ignition of the cathodes.